1. Field of the Invention
This invention relates to differential pressure transmitters.
2. Discussion of the Background
The differential pressure transmitter generally has a body case in which a pressure transmission passage if formed. The pressure transmission passage has both ends open at the outer surface of the body case. One of the pressure transmission passage is closed by a high-pressure side, pressure-receiving diaphragm for receiving a pressure Pa to be detected, and the other end is closed by a low-pressure side, pressure-receiving diaphragm for receiving a pressure Pb to be detected. The pressure transmission passage is filled with a pressure transmission medium, e.g., silicone oil. Pressures applied to the pressure receiving diaphragms are transmitted to the pressure transmission medium. The body case also has a differential-pressure/displacement element, e.g., pressure-receiving diaphragm, which partitions the pressure transmission passage into a high-pressure side chamber and a low-pressure side chamber. This pressure-receiving diaphragm is displaced according to the pressure difference between the chambers. In the body case is further disposed a pressure-sensing element, which has a pressure-receiving surface for receiving the pressure in the high-pressure side chamber and a pressure-receiving surface for receiving the pressure in the low-pressure side chamber. The pressure-sensing element detects the difference between the pressures Pa and Pb and converts it into an electric signal.
When such a differential pressure transmitter is installed in a plant or the like, not only a static pressure but also pressures varying at various frequencies (hereinafter referred to as pulsating pressures) are applied to the high-pressure side and low-pressure side pressure-receiving diaphragms. The pulsating pressure is so great that it can not be ignored as compared to the differential pressure to be detected, and can cause variations in the output of the differential pressure transmitter even in an ordinary case. Therefore, it is necessary to remove the pulsating pressure. Among the known means for removing the pulsating pressure is one in which an electric filter is connected to the output terminal of the pressure-sensing element, and another in which a throttle is provided on the fluid side for removing the pulsating pressure by making use of the displacement of the volume of pressure transmission medium in correspondence to a change in the pressures applied to the differential pressure transmitter. The former means is simple, but in this case pulsating pressures are applied to the interior of the differential pressure transmission apparatus and also to the pressure-sensing element. For this reason, it is desirable from the standpoint of the life of the differential pressure transmitter to either use the latter means or the two means in combination.
As the pulsating pressure removing means with a throttle provided on the fluid side, there is one in which a throttle is provided in a pipe running from a process fluid line to the inlet of the differential pressure transmitter. In this case, however, depending on the kind of measurement fluid, clogging is possible and also the extent of the throttling and viscosity of the measurement fluid are influenced by the extent to which air bubbles in the pipe have been removed. Further, it has been the practice to provide the inner wall of the pressure transmission passage in a body case with a throttle section consisting of a protuberance. When filling the pressure transmission passage with the pressure transmission medium, the medium is charged into the passage in a vacuum and then the air is purged. Where a throttle section is provided in the pressure transmission passage, however, it is difficult to smoothly carry out the charging of the medium and air purging, and in some cases the performance of the differential pressure transmission apparatus is deteriorated by the remaining air bubbles. Further, there has been well-known a structure whose throttle is externally variable. In this case, however, the throttle is varied while maintaining the seal between the inside and outside of the body case. Therefore, the overall construction is complicated, leading to a high cost. In addition, reliability is reduced with the increase in seal sections.